Structure,Synthesis, and Properties of Some Persubstituted 1,2-Dintroethanes. In Quest of Nitrocyclopropyl-Anion Derivatives

Attempts to deprotonate nitrocyclopropane led to solutions which showed strong ESR. signals (Fig. 1)and from which 1-nitro- l′-nitroso-bicyclopropyl ( 3 ) and 1,1′-dinitro-bicyclopropyl ( 2 ) were isolated. The activation energy for rotation about the central C, C-bond of 2 is estimated to be about 12 kcal/mol (¹ H-NMR. spectra in Fig. 2). In contrast, the oven-chain analogue 2,3-dimethyl-2,3-dinitrobutane ( 1 ) shows a methyl singlet down to ?70° C. Low-temperature X-ray analyses of 1, 2, 3 , and also of 1,1′-dinitro-bicyclobutyl ( 4 ) show that all four molecules have gauche-conformations but reveal striking structural differences between the openchain and the cyclic derivatives (Fig. 4–6): the central C, C-bond is long in 1 (1.575 Å), short in 2 (1.479 Å); the C, N-bonds are long in 1 (1.549 Å), short in 2 (1.488 Å); the orientation of the nitro groups is bisected in 2 and perpendicular in 1. The crystal structure of the nitro-nitroso compound 3 is isomorphous with that of the dinitro compound 2 and thus disordered (Fig. 15–16). The effect of the nitro group as π-electron acceptor on the molecular conformations and bond lengths is discussed. From analysis of the anisotropic vibrational parameters of 2 the root-mean-square librational amplitude of the nitro groups about their C, N-bonds is estimated to be about 5.8° at 95 K, corresponding to a rotational barrier of about 9 kcal/mol, i. e. the same order of magnitude as the NMR. estimate of about 12 kcal/mol for C, C-rotation.